Ehrlichiosis

Definition

Ehrlichiosis is an infectious disease transmitted by the bite of a tick.

Alternative Names

Human monocytic ehrlichiosis; HME; Human granulocytic ehrlichiosis; HGE; Human granulocytic anaplasmosis; HGA

Causes, incidence, and risk factors

Ehrlichiosis is caused by bacteria that belong to the family called Rickettsiae. Rickettsial bacteria cause a number of serious diseases worldwide, including Rocky Mountain spotted fever and typhus. All of these diseases are spread to humans by a tick, flea, or mite bite.

Scientists first described ehrlichiosis in 1990, and have identified two types in the United States:

• Human monocytic ehrlichiosis (HME) is caused by the rickettsial bacteria called Ehrlichia chaffeensis.
• Human granulocytic ehrlichiosis (HGE) is also called human granulocytic anaplasmosis (HGA). It is caused by the rickettsial bacteria called Anaplasma phagocytophilum (once called Ehrlichia equi or Ehrlichia phagocytophila).

Ehrlichia bacteria can be carried by the Lone Star tick, the American dog tick, and the deer tick, which can also cause Lyme disease.

In the United States, HME is found mainly in the southern central states and the Southeast. HGE is found mainly in the Northeast and upper Midwest.

Risk factors for ehrlichiosis include:

• Living near an area with a lot of ticks
• Owning a pet that may bring a tick home
• Walking or playing in high grasses

Symptoms

The time between the tick bite and when symptoms occur is about 7 - 9 days. This is called the incubation period.

Symptoms may seem like the flu (influenza), and may include:

• Chills
• Fever
• Headache
• Muscle aches
• Nausea

Other possible symptoms:

• Diarrhea
• Fine pinhead-sized areas of bleeding in the skin (petechial rash)
• Flat red rash (maculopapular rash)
• Malaise

A rash appears in less than half of the cases. Sometimes, the disease may be mistaken for Rocky Mountain spotted fever. The symptoms are often quite general, but patients are sometimes sick enough to see a doctor.

Signs and tests

The doctor will do a physical exam and check your vital signs, including:

• Blood pressure
• Heart rate
• Temperature

Other tests include:

• Complete blood count (CBC)
• Granulocyte stain
• Fluorescent antibody test